[G-CPTN]

The video:- http://www.youtube.com/watch?v=ui63-Zjbcvo
shows (IMO) London trams operating without overhead wires with a third rail visible between the tracks.
I'd never heard of this system before (though I am not a tram enthusiast).
A search on Googoo revealed a new third-rail system being installed in Boulogne in 2004 (though there is a sophisticated modern-technology safety system):-

A distinguishing feature of the Bordeaux network is the absence of overhead wires in the central area. When the original tram network was built, the city authorities adopted a conduit power supply for the tram network, to protect the visual image of the city centre as laid out by Baron Haussmann at the end of the 19th century, and this lasted until closure in the 1950s. When CUB decided in 1996 to reintroduce light rail, bidders were again invited to put forward proposals for propulsion systems which would avoid the need for wires.

Options considered included battery, flywheel or diesel powerpacks, but all of these would have posed operating restrictions due to the limited power availability. Eventually the choice fell on the Innorail APS (Alimentation par Sol) ground-level power supply system. Innorail was originally formed as a wholly-owned subsidiary of electrification specialist Spie Enertrans, but when that firm was acquired by Amec it sold its shares in Innorail to Alstom.

Getting approval of the APS equipment under new French safety-case regulations proved one of the time-critical factors in commissioning the network, according to Alstom Project Director Hubert Peugeot and Innorail Managing Director Antoine Picard. Test running was undertaken on an isolated 500m section in Lormont, which could be physically sealed off from other road users. After formal tests and an independent audit, the safety case was finally signed off on October 21, allowing commissioning trials on the remainder of the network to start barely two months before 'T-Day'.

Even before APS received its safety approval, it had already become clear that all three routes would not be ready in time. The CUB tramway project team decided to concentrate all efforts on completing Line A, and postponed the opening of Lines B and C to February. However, in early October one car was towed through the uncompleted junctions, across the city centre and out to Pessac. This enabled the start of gauging runs and commissioning on the overhead-equipped section of Line B from the outer terminus inwards.
Digital radio switching

APS relies on digital radio signals to energise short sections of a central third rail under the vehicles, providing a full 750V DC supply to each car as it passes. The first phase of the Bordeaux network has been fitted with around 10·5 km of APS, split into five sections over all three routes. As well as the city centre tracks, short lengths of APS have been installed on both eastern branches of Line A at the wish of the local communities. It is also being used on part of Line B, where the fire brigade wanted unfettered access to buildings along a narrow street.

The centre rail has two raised contact strips on the head, about 20mm wide, separated by a 10mm recess. The rail is laid slightly higher than the running rails, so that the pick-up shoes pass above the running rails at pointwork. The contact rail is split into 8m sections, separated by 3m glass fibre inserts to form an isolation gap. The power is fed via 980 switching boxes set into the track, each of which powers two adjacent 8m lengths.

The two sets of pick-up shoes on each car are carried at opposite ends of the single unpowered bogie, immediately beneath the pantograph. The shoes are spaced 3·2m apart to bridge the insulated gaps and ensure continuity of supply. The cars were initially fitted with cast iron pick-up shoes to help polish the contact surface of the newly-laid rails. After a few months the shoes will be replaced by softer carbon pick-ups to avoid excessive wear.

Changeover between overhead catenary and APS modes takes place while the vehicle is stationary at a stop. The driver presses a button to initiate an automated sequence which lowers the pantograph and extends the parallelogram-mounted shoes to engage the contact rail. Each car is also fitted with emergency batteries which will allow it to run for up to 1000m should it become isolated from the third rail or overhead supply.

To energise the ground-level supply, each LRV operating in APS mode repeatedly broadcasts an 8-bit digital message to an antenna under the contact rail. The radio message is broadcast as a conical signal no more than 100mm in diameter at ground level, to avoid any risk of cross-energising the adjacent track. Only when a switching box has received and identified the 8-bit message four times in quick succession will the relays energise the appropriate section of rail.

The positioning of the shoes and radio antennae is designed to ensure that the rail is only live underneath a vehicle. Changeover contacts on the track-mounted switchgear earth the centre rail to the running rails at all other times, to avoid any risk of a residual potential.

In the event of any fault, each switchbox is designed to fail safe and isolate both rail sections. An alarm will be triggered at the control centre, but the service should be able to continue with the cars coasting across the gap. The faulty switchbox will be replaced during the night, with quick-release connections for a rapid change-over. Defective boxes will be returned to the supplier's workshop for servicing.

APS will be used on 3·5 km of the Phase 2 extensions, including the bulk of the Mériadeck - CHR section where the fire brigade have again requested clear access to the hospital buildings.

The contract price for the APS equipment on Phase 1 was €1·5m per track-km, which is around three times the price of the conventional overhead sections. For the CHR extension the price has increased to €2m, although some observers suggest that the true cost may be nearer €3m. Picard says the price is likely to come down in the longer term as the technology is used more widely, and notes that Nice and Paris are among several cities that have already expressed interest in using a third-rail supply.

(from:- http://www.railwaygazette.com/news_v...e_garonne.html )

Another overhead-wire-free system is described here:-
http://www.railwaygazette.com/news_v...tion_tram.html

Sections of the Braunschweig system are also three-rail:-
http://www.railway-technology.com/pr.../braunschweig/
but these are all modern schemes.

Did London really have three-rail trams in the 1930s?

It seems so:-

The Glamorous Electric Trams
This heralded a major expansion into electric tramways. The London County Council (LCC) began taking control of the horse tramways in London in 1896, and by 1899 had taken over the principal lines in south London. They now began an extensive programme of electrifying the old routes. For this they adopted the 'conduit' system of supplying power by means of a live rail buried beneath the road surface, rather than the cheaper, but unsightly, overhead lines. The first of these new electric-powered conduit lines opened on 15 May 1903. It ran from Blackfriars and Westminster to one of the LCC's new cottage estates in Tooting, south London.

(from:- http://www.storyoflondon.com/modules...rder=0&thold=0 )

More on ground-level supply:-
http://en.wikipedia.org/wiki/Ground-level_power_supply

and conduit-collection:-
http://en.wikipedia.org/wiki/Conduit_current_collection

The last (London) tram was withdrawn in June 1952.
The last conduit line in Paris closed in 1936, while the last Bordeaux conduit car ran in 1953.
The conduit systems in Berlin, Vienna and Budapest were very short-lived. All three were replaced by overhead working before World War I.